Configurations and Methods of Electrochemical Lead Recovery from Contaminated Soil

ABSTRACT

A soil remediation system includes an electrochemical cell that is configured to provide increased mass transfer and a decreased diffusion layer between the electrodes to thereby allow formation of a homogenous lead deposit that is substantially free of dendrite formation and easily removed.

This application claims the benefit of U.S. Provisional PatentApplication with the Ser. No. 60/462,160, filed Apr. 10, 2003, which isincorporated by reference herein.

FIELD OF THE INVENTION

The field of the invention is electrochemical soil remediation, andespecially as it relates to electrochemical recovery of lead from alead-complex solution from contaminated soil.

BACKGROUND OF THE INVENTION

There are various methods of soil remediation of lead contaminated soilknown in the art, however, all or almost all of them exhibit significantdisadvantages. For example, lead can be removed from soil in situ usinga complexing agent (e.g., EDTA: ethylenediamine tetraacetic acid) asdescribed in U.S. Pat. No. 5,316,751. Where desired, alternativebiodegradable complexing agents may be employed as described in U.S.Pat. No. 6,264,720. Lead-EDTA and other lead complexes are often highlystable and form relatively quickly over a relatively wide pH range.However, where such complexes are formed in situ, great care must betaken to avoid mobilizing the solubilized lead away from the site ofcontamination (e.g., into an aquifer).

Alternatively, lead may be electrochemically isolated from soil in aslurry by positioning the electrodes into the slurry as described inU.S. Pat. No. 4,193,854, or lead may be isolated from soil directly byplacing the electrodes into the soil as described in U.S. Pat. Nos.5,137,608 and 5,458,747. While such electrolytic methods oftensignificantly reduce the risk of inadvertent contamination ofuncontaminated areas, various difficulties remain. Among other things,and depending on the lead concentration, soil composition, and/orconductivity of the soil, electrochemical recovery may not beeconomically attractive. Moreover, electrochemical lead removal may notbe practicable where the contaminated area is relatively large.

In still further known methods, lead can be extracted from a lead-EDTAsolution that is electrolyzed to plate lead on a cathode. However, insuch configurations, EDTA is typically electrochemically degraded at theanode, which renders such systems cost-ineffective. Moreover, as theconcentration of the lead-EDTA complexes decreases, low mass transferconditions are likely to develop and consequently electrolysis wouldoperate under current limiting conditions. Such conditions will not onlyrender electrolysis cost-ineffective, but also lead to generation ofhydrogen, which is highly undesirable. Still further such conditionstypically lead to dendritic lead deposits which are less useful and aredifficult to recover.

Thus, although there are numerous configurations and methods for leadrecovery are known in the art, all or almost all of them suffer from oneor more disadvantages. Still further, disposal of the processing fluidsand removal of the residual lead and EDTA from soil is oftenproblematic. Therefore, there is still a need to provide improvedcompositions and methods for lead recovery from contaminated soil.

SUMMARY OF THE INVENTION

The present invention is directed to configurations and methods of leadrecovery from an electrolyte in which lead is electrochemically platedfrom a complex formed between lead and a complexing agent in anelectrochemical cell that provides forced flow of the electrolytebetween the electrodes to provide increased mass transport, loweroperating costs, and more effective removal of the target metal. Thecell is preferably configured to enable protection of the organiccomplexing agent from oxidation at the anode so that the complexingagent can be recycled to the soil many times.

In another aspect, the target metal in the process fluids of the firstcell system is removed in a second cell to a sufficiently low level thatallows disposal of the electrolyte into the sewer without violatingdischarge limits. Such second cells are typically of specific value atthe end of the treatment process for the site. Contemplatedconfigurations generally allow removal of the target metal from soil tomeet leach tests levels demanded by the Japanese environmentalguidelines for the complexing agent and the target metal (which iscurrently more stringent in the US or Europe).

In one especially preferred aspect, contemplated electrolytic cellsinclude an anode, a cathode, and an electrolyte comprising lead incomplex with a complexing agent. A pump is fluidly coupled to theelectrolytic cell and moves the electrolyte between the anode andcathode at a predetermined flow velocity, wherein the anode and thecathode are positioned relative to each other such that a flow path isformed between the anode and cathode from which lead is deposited ontothe cathode at non-current limiting conditions at the flow velocity.

In such configurations, it is especially preferred that the cathode isdisposed in a cathode container that contains the electrolyte, and/orthat the anode is disposed in an anode container that includes ananolyte that is circulated between the container and an anolytecirculation tank, wherein the anode container is at least partiallydisposed in the cathode container. Further preferred anode containersinclude a separator (e.g., diaphragm or ion exchange polymer), and it isalso contemplated that the cathode container is in fluid communicationwith a tank that contains the electrolyte.

Thus, in another aspect of the inventive subject matter, an electrolyticcell will include (1) a first container that contains an acidiccatholyte comprising lead in complex with a complexing agent, wherein acathode is at least partially disposed within the catholyte, (2) a pumpthat moves the catholyte across the cathode at a predetermined flowvelocity, and (3) a second container that contains an anolyte, whereinthe second container is at least partially disposed in the catholyte andcomprises a separator that separates the catholyte from the anolyte,wherein the second container further comprises an anode, and wherein thecathode and the second container are positioned relative to each othersuch that a flow path between the second container and cathode is formedfrom which the lead is deposited onto the cathode at non-currentlimiting conditions at the predetermined flow velocity.

The first container in such electrolytic cells may advantageouslyinclude a first opening that receives the catholyte and a second openingthat discharges the catholyte after the catholyte has contacted thesecond container, and it is further preferred that the first containeris at least partially disposed in a tank that receives the catholytefrom the second opening and that provides the catholyte to the firstopening. While not limiting to the inventive subject matter, it isgenerally preferred that the acidic catholyte comprises sulfuric acid,that the complexing agent is ethylenediamine tetraacetic acid, and/orthat the cathode comprises titanium and the anode comprises lead oriridium oxide coated titanium.

In further contemplated aspects, the anolyte (preferably comprisingsulfuric acid) is provided to the second container from an anolytecirculation tank, and especially suitable separators include a diaphragmor an ion exchange polymer (e.g., Nafion). With respect to theconcentration of lead in the electrolyte, it is preferred that the lead(preferably in complex with the complexing agent) has a concentration ofless than 5000 ppm, more preferably less than 500 ppm, and mostpreferably less than 250 ppm.

Especially preferred flow velocities of the catholyte across the cathodeare those that provide a Reynolds number (Re) of above 2000. Thus,exemplary preferred flow velocities are at least 0.05 n/sec (at a gap ofabout 2.54 cm), and more preferably at least 0.08 m/sec (at a gap ofabout 2.54 cm). Therefore, particularly preferred non-current limitingconditions are typically proportional to the metal concentration and Re.

In yet another especially preferred aspect, contemplated electrolyticcells may comprise an electrolyte reservoir that contains an electrolytein which lead is complexed with a complexing agent. A first container ispreferably at least partially disposed within the electrolyte reservoir,wherein the first container further includes a cathode, a first openingthat receives the electrolyte from the electrolyte reservoir, and asecond opening that provides the electrolyte back to the electrolytereservoir, and a second container is at least partially disposed withinthe first container, wherein the second container further includes ananolyte and an anode, and wherein the anolyte in the second container isseparated from the electrolyte in the first container by a separator. Apump is fluidly coupled to the electrolyte reservoir and moves theelectrolyte from the electrolyte reservoir to the first container viathe first opening at a rate effective to prevent formation of adiffusion layer in a flow path that is formed between the secondcontainer and the cathode.

Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention, along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic perspective view of an exemplary electrolytic cellaccording to the inventive subject matter.

FIG. 2 is a schematic detail view of the exemplary electrolytic cell ofFIG. 1.

FIG. 3 is a picture of a cathode of an electrolytic cell according tothe inventive subject matter showing a partially-scraped lead plate.

DETAILED DESCRIPTION

The inventors have discovered that lead can be effectively plated, andmost preferably as a smooth film from a solution comprising very lowconcentrations of lead, which is preferably in complex with a chelatingagent. While lead deposition in form of a smooth layer has been knownfor high lead concentrations (typically 1M to 2M, and even higher),known configurations and methods, and especially under non-currentlimiting conditions, failed to remove lead from an electrolyte where thelead was present in low concentrations (i.e., less than 5000 ppm, moretypically less than 500 ppm, and most typically less than 250 ppm). Itshould therefore be particularly recognized that contemplatedconfigurations may be employed in remediation where the concentration oflead (or other metals, see below) is relatively low, and especiallywhere the metal is to be removed in a commercially and/or technicallyattractive form (e.g., with a purity of at least 99%).

Contemplated electrolytic cells include those having a configurationthat provides high mass transport conditions between the anode andcathode. Viewed from another perspective, the inventors discovered acell configuration in which lead is electrolytically recovered atrelatively low concentrations under non-current limiting conditions byavoiding formation of an inhibiting diffusion layer.

As used herein, the term “anode” refers to the electrode in theelectrolytic cell at which oxidation occurs when current is passedthrough the electrolytic cell. Therefore, under typical operatingconditions, molecular oxygen (O₂) is generated at the cathode fromwater. As also used herein, the term “anolyte” refers to the electrolytethat contacts the anode.

As used herein, the term “cathode” refers to the electrode in theelectrolytic cell at which reduction occurs when current is passedthrough the electrolytic cell. Therefore, under typical operatingconditions, elemental metals are plated onto the cathode from ionicmetals (which may or may not be complexed with a chelating agent). Asfurther used herein, the term “catholyte” refers to the electrolyte thatcontacts the cathode. In most embodiments according to the presentinventive subject matter, the anolyte is separated from the catholytevia a separator that allows migration of a charged species from theanolyte to the catholyte (and vice versa), but is otherwise impermeablefor the anolyte and catholyte.

As still further used herein, the term “non-current limiting condition”refers to a condition in which a metal, and most typically lead, isdeposited from an electrolyte onto a cathode before the metal depositionreaches current limiting condition (i.e., a condition where increase ofthe cathode potential fails to proportionally increase the rate ofdeposition). Viewed from another perspective, deposition of the metal atthe cathode occurs before complete mass transport control sets in (i.e.,the rate of convective diffusion determines the rate of deposition). Itshould be particularly noted that as a consequence of metal depositionat non-current limiting conditions, the metal, and especially lead willplate at the cathode in form of a smooth film as opposed to a powdery,grainy, or dendritic deposit as would be the case at current limitingconditions. The term “smooth film” as used herein refers to a metaldeposit that has an metal oxide content of less than 1% (e.g., less than1% lead oxide in deposited lead) and an impurities content of less than1% (e.g., less than 1% calcium, magnesium, sulfides, and/or salts indeposited lead).

As yet further used herein, the term “diffusion layer” refers to aconcentration gradient of lead within the electrolyte, wherein theconcentration of lead ions is lowest at or near the cathode (i.e.,within less than 5 mm) and increases as the distance from the cathodeincreases, and wherein deposition of the lead onto the cathode at theconcentration of lead at or near the cathode is at current limitingconditions. Thus, as used herein, the term “prevent formation of adiffusion layer” is synonymously used with the term “prevent currentlimiting conditions”.

In one especially preferred aspect of the inventive subject matter, asdepicted in FIG. 1, an electrolytic cell 100 has a nested andself-contained configuration in which an catholyte recirculation tank140 includes a catholyte container 110 that in turn includes an anolytecontainer 120.

With further reference to FIG. 1, the catholyte container (firstcontainer) 110 includes an acidic catholyte (not shown), wherein thecatholyte comprises lead in complex with a complexing agent. A first anda second cathode 112A and 112B are partially disposed within thecatholyte, wherein the catholyte enters the cathode container via firstopening 114A and leaves the cathode container via overflow at the opentop (second opening 114B) of the cathode container. The overflowingcatholyte is received by catholyte recirculation tank 140, from whichpump 130 transports the catholyte back into the catholyte container viathe first opening 114A.

Disposed within the catholyte container is an anolyte container (secondcontainer) 120 that contains anode 122 (not shown) and an acidic anolyte(not shown), which is circulated via a pump 124 to and from an anolytecirculation tank 126. The anolyte container further includes a separator128 that is permeable for ions and contacts both the anolyte andcatholyte.

FIG. 2 provides a schematic cross sectional detail view of theelectrolytic cell of FIG. 1, in which the electrolytic cell 200 has acatholyte recirculation tank 240 with an outlet 242 that providescatholyte to the pump 230. At least partially disposed within thecatholyte recirculation tank 240 is the catholyte container 210 thatincludes a first opening 214A through which the catholyte containerreceives the catholyte from the pump 230, and a second opening (here:open top) 214B from which the catholyte is fed to the catholyterecirculation tank 240 after the catholyte has contacted the cathodecontainer 210. A pair of cathodes (cathodes 212A and 212B) is further atleast partially disposed in the catholyte (within the cathode container210).

Still further and at least partially disposed in the cathode container210 is anode container 220 that includes an anode 222 at least partiallydisposed in the anolyte (not shown). The anode container 220 has aseparator 224 (most preferably a NAFION™ [poly(tetrafluoroethylene)membrane, commercially available from DuPont] membrane) that separatesthe anolyte from the catholyte. A flow path 250 is formed between thecathodes 212A and 212B and the separators 224 of the anode container,wherein lead deposited from the flow path onto the cathodes is depictedas small triangles.

It should generally be appreciated, however, that numerous modificationsto the above described systems may be made. For example, while lead is apreferred metal for electrolytic recovery, it is also contemplated thatnumerous alternative metals (and especially heavy metals) are suitablefor use in conjunction with the teachings presented herein. Therefore,contemplated metals also include zinc, copper, cadmium, mercury, nickel,etc. It is further contemplated that the metal may occur bound to asolid phase (e.g., ionically bound to soil), in ionic form with acounter ion (e.g., as a salt deposit), or dissolved as an ionic species.

In especially preferred aspects of the inventive subject matter, themetal is solubilized into a liquid, and most preferably an electrolyteby leaching/isolating the metal from its location (e.g., from a solidphase or salt deposit) using a leaching agent. The term “leaching agent”as used herein is interchangeably used with the terms “complexing agent”and “chelating agent” and refers to a molecule that binds a metal ionvia one or more (typically non-covalent) complex bonds to form ametal-complexing agent complex (e.g., lead that forms with EDTA alead-EDTA complex). Further contemplated manners of solubilizing a metalinclude salt formation (e.g., metal/methanesulfonate salt).

Consequently, it should be recognized that the nature of the complexingagent may vary considerably, and all known complexing agents for metalions are deemed suitable for use herein. Thus, especially preferredcomplexing agents include monodentate, bidentate, tetradentate, andpolydentate complexing agents, which may or may not exhibit selectivityfor a particular metal ion. For example, where the chelating agentcomprises an organic acid, suitable complexing agents include citrate,poly(aspartate), EGTA, EDTA, etc. On the other hand, non-acid complexingagents may include those in which a nitrogen (or other non-carbon) atomin an aromatic ring is employed to bind the metal ion (e.g., nickelbound by nitrogen of an imidazole ring).

Where the metal ion is isolated from soil, and especially where themetal ion is lead, it should be recognized that the nature of thecomplexing agent may also vary depending on the type of soil (e.g., dueto the presence of other ions that may potentially compete with thecomplexing agent, or due to the pH in the soil). For example, where thesoil is a non-clay soil, EDTA may be employed as the complexing agent.On the other hand, where lead is to be isolated from a clay or clay rich(typically >20% clay) soil, methane sulfonic acid or sulfamic acid maybe employed as a complexing agent. In such cases, it should further berecognized that acidity may be provided by the chelating agent (e.g.,via deprotonation of free methane sulfonic acid). Furthermore, it shouldbe recognized that the concentration of the complexing agent may varyconsiderably, and it is generally contemplated that the complexing agentmay be present in sub-stoichiometric quantities, stoichiometricquantities, or in super-stoichiometric quantities. However, it isgenerally preferred that the chelating agent is present in at leaststoichiometric quantities.

Therefore, contemplated electrolytes (and particularly contemplatedcatholytes) will vary substantially and the particular composition willgenerally depend on the metal and complexing agent of choice (supra).Still further, it is generally preferred that the pH of the catholyte isless than 7.0, but higher pH values are not excluded.

In a particularly preferred aspect of the inventive subject matter, thecatholyte is generated by contacting metal contaminated soil with asolution that comprises the chelating agent at a suitable pH. In suchconfigurations, the contaminated soil may be excavated and then flushed(batch-wise or continuously) with the solution that comprises thechelating agent. Alternatively, the soil may also be contacted in situwith the solution that comprises the chelating agent to generate thecatholyte. The so generated catholyte may then be further processedbefore use in electrolytic recovery of the metal, and especiallycontemplated processing steps include filtration, acidification oralkalinification for adjustment of pH, addition of chelating agent,salt, or other component.

With respect to the anolyte, it is generally preferred that the anolyteis an aqueous acidic solution (e.g., sulfuric acid). However, inalternative aspects the nature and composition of the anolyte may varysubstantially. For example, suitable anolytes may be neutral (i.e., pHbetween about 6.5 to about 7.5), or include a solvent other than water.Still further, suitable anolytes may also include one or more species ofsalt to increase conductivity or to enhance other desirable properties.Numerous anolytes for metal deposition electrolysis are known in theart, and all of them are considered suitable for use herein.

In further preferred aspects of the inventive subject matter, thecatholyte recirculation tank has a capacity of at least three times thevolume of the container and further includes at least one port throughwhich catholyte is withdrawn (that previously contacted the catholytecontainer and/or the cathode). In alternative aspects, the configurationof the catholyte recirculation tank may vary substantially. For example,the volume of the catholyte recirculation tank may be less than threetime the volume of the catholyte container where the volume of catholyteis relatively low, or where multiple catholyte recirculation tanks areemployed. Alternatively, and especially where the catholyte is generatedin situ, the catholyte recirculation tank may be in form of a pipelinethat is fluidly coupled to the site where the catholyte is generated. Onthe other hand, it should also be appreciated that the catholyte may begenerated from contaminated soil in the catholyte recirculation tank. Insuch (and other) configurations, it should be recognized that the volumeof the catholyte recirculation tank may be significantly higher thanthree times the volume of the cathode container. Thus, viewed fromanother perspective, suitable catholyte recirculation tanks willgenerally be fluidly coupled to the cathode container and at leastreceive catholyte from the cathode container, and more preferably atleast partially include the cathode container.

Similarly, the configuration of contemplated catholyte containers mayvary considerably. However, it is generally preferred that the cathodecontainer receives catholyte from the catholyte recirculation tank andincludes (a) at least one opening that provides the catholyte (aftercontact with the cathode) to the catholyte recirculation tank, and (b)at least one cathode. In further especially preferred aspects, it iscontemplated that the cathode container is configured to at leastpartially fit within the catholyte recirculation tank, and that at leastpart of the catholyte travels upwardly along a flow path (infra) that isformed between a cathode and the anode container. Thus, suitable cathodecontainers will include one or more ports in a lower portion (i.e.,below the midpoint of the container) through which catholyte enters thecathode container, and one or more openings (and most preferably an atleast partially open top as shown in FIG. 1) in an upper portion (i.e.,above the midpoint of the container) through which catholyte leaves thecathode container.

Alternative cathode containers may have numerous configurations otherthan those describes above so long as such cathode containers receivecatholyte from the cathode recirculation tank and provide catholyte backto the catholyte recirculation tank after that catholyte has flownthrough the cathode container. For example, suitable cathode containersmay have a cylindrical shape where the catholyte recirculation tank isalso cylindrical. Furthermore, where appropriate, more than one cathodecontainer may be at least partially positioned within the catholyterecirculation tank. In such configurations, it should be recognized thatthe catholyte may flow from one cathode container to the next catholytecontainer, and from the last cathode container back to the catholyterecirculation tank in a serial configuration. Alternatively, thecatholyte may also flow from each cathode container back to thecatholyte recirculation tank in a parallel configuration.

It is generally further preferred that suitable cathode containers willinclude two cathodes, wherein the two cathodes are separated from eachother by the anode container. Thus, each cathode container will includeat least two distinct flow paths for the catholyte (infra). With respectto the cathode material, it is contemplated that all conductivematerials are appropriate so long as such materials will allowdeposition of the metal onto the cathode. However, it is generallypreferred that the cathode comprises, and most preferably is fabricatedfrom titanium. Still further contemplated alternative cathode materialsinclude carbon, stainless steel, titanium, nickel-plated iron, preciousmetal coated titanium, conductive plastics, lead, and all reasonablecombinations and alloys thereof.

It is generally preferred that the anolyte container is configured suchthat (a) the anolyte container can be juxtaposed to at least onecathode, and more preferably positioned between a pair of cathodes, and(b) that the anolyte container forms a flow path in cooperation with atleast one cathode in the cathode container. Especially preferred anodecontainers will include an anode that is at least partially disposedwithin an anolyte, wherein the anolyte is preferably circulated betweenthe anode container and an anolyte recirculation tank. It should beespecially recognized that such configurations advantageously allow forrelease of oxygen gas generated at the anode as well as for cooling toat least some degree.

Suitable anode containers further include at least one, and morepreferably at least two separators that separate the anolyte from thecatholyte while allowing the flow of charged species, and especially theflow of cations and protons. Therefore, particularly suitable separatorsinclude diaphragms and ion exchange polymers (e.g., NAFION™) well knownin the art, and all of such separators are considered suitable for usein conjunction with the teachings presented herein.

With respect to the anode it is generally contemplated that allconductive materials are appropriate so long as such materials willallow electrolytic conditions that provide a current suitable fordeposition of the metal onto the cathode in the cathode container.However, it is generally preferred that the cathode comprises, and mostpreferably is fabricated from lead or iridium oxide coated titanium.Still further contemplated alternative anode materials include carbon,platicarbon, platinized titanium, stainless steel, nickel, lead, and allreasonable combinations and alloys thereof.

It should be especially appreciated that the flow channel incontemplated electrolytic cells is formed between the cathode and theanode container and configured such that mass transport is increased atthe electrode interface by increasing turbulence and/or flow velocity.Viewed from another perspective, the flow channel in contemplatedelectrolytic cells has a configuration such that an otherwise formingdiffusion layer is disturbed, or even completely eliminated by theflowing electrolyte. The inventors discovered that without suchconfigurations, the concentration of the metal in the catholyte woulddecline at the cathode surface as electrolysis increasingly depletes theconcentration of metal, which in turn would results in current limitingconditions and formation of hydrogen gas. Further suitable electrolyticcells are described in our provisional patent application with the Ser.No. 60/485,879, which was filed Jul. 8, 2003, and which is incorporatedby reference herein.

Consequently, and among other advantages, contemplated configurationswill provide a substantially increased current efficiency over knownconfigurations (typically static systems or systems with a stir bar) andremoval of metal ions from the catholyte below previously achievedconcentrations at comparable energy costs. Moreover, the metal andespecially lead deposited onto cathodes in contemplated systems willform a smooth film which can be easily removed (typically peeled) fromthe cathode. In contrast, electrodeposition in known electrochemicalcells will typically result in grainy, powdery deposits, and moretypically result in dendrite formation eventually leading to puncture ofthe separator or short-circuits in systems without separators.

It should be recognized that there are numerous manners of forming aflow channel, and all of such manners are contemplated herein. However,it is generally preferred that the flow channel is directly formedbetween the cathode and the anode container as depicted in FIGS. 1 and2. Here, an upward flow path is formed by supplying catholyte to thebottom of the catholyte container and placing the cathodes and anodecontainer such that a significant portion (i.e., at least 25 vol %, moretypically at least 50 vol %, and most typically at least 80 vol %) ofthe catholyte entering the cathode container will pass between thecathode and the anode container and exit the open top of the cathodecontainer as overflow.

In alternative aspects and where appropriate, it is contemplated thatthe cathode and/or anode container (including the separator) may furthercomprise protrusions that will increase and/or induce turbulent flowbetween the anode container and the cathode. Alternatively, funnels orjets may be directed between the cathode and anode container to disturbformation of a diffusion layer. In still further contemplatedembodiments, it should be recognized that numerous other flow paths maybe formed, and all of such flow paths are deemed suitable so long assuch flow paths will prevent formation of a diffusion layer at apredetermined flow velocity. Prevention of formation of a diffusionlayer can be ascertained by a person of ordinary skill in the art in arelatively simple manner by visual confirmation that the metal depositedis in form of a smooth film, or by observation that the metal isdeposited under non-current-limiting conditions at a given flowvelocity.

With respect to the flow velocity, it is generally contemplated that theflow velocity will be at least in part determined by the current densityand/or concentration of the metal in the catholyte. Therefore, numerousflow velocities are deemed suitable, and it should be recognized that aperson of ordinary skill in the art will be readily able to determinethe flow velocity on an empirical basis. Furthermore, it should berecognized that the flow velocity may be adjusted over the course of anelectrolytic recovery of the metal.

Viewed from another perspective, it should be recognized that thecathode and the anode (or anode container) are positioned relative toeach other such that the flow velocities in the flow path provides for aReynolds number (Re) of at least 2000. Therefore, the limiting currentdensity in the flow path will be generally proportional to the metalconcentration and the Re.

Observations and Experimental Data

The relationship between electrical current and cathode potential formetal deposition can be experimentally determined and is schematicallydepicted for copper deposition from an acid sulfate solution in Graph 0below. As the cathode potential is made more negative than the opencircuit potential, the current (and therefore the rate of copperdeposition) increases. At sufficiently negative potential, the rate ofmetal deposition reaches a maximum in the limiting current (I_(L))plateau region. Here, the rate of cupric ion removal is dominated by therate at which copper ions are supplied to the cathode (typically byconvective-diffusion), which is also known as complete mass transportcontrol. If the potential is too negative, the current once again risesdue to secondary reactions (e.g., hydrogen evolution).

Under completely mass transport controlled conditions, the rate of metalion removed is thus given by Faraday's laws of electrolysis, and can beexpressed as

$\begin{matrix}{\frac{w}{t} = \frac{\phi \; {IM}}{zF}} & (1)\end{matrix}$

where w is the mass of metal, t is the time, φ is the cathode currentefficiency, I is the current, M is the molar mass of metal, z is thenumber of electrons and F is the Faraday constant. Consequently, itshould be appreciated that high values of current efficiency should bemaintained while limiting current density and cathode area to secure ahigh rate of metal ion removal from an electrolyte. On the other hand,for a batch electrolyte of volume V, the change in molar concentrationof metal ions due to electrochemical reactions, Δc, may be therefore beexpressed as:

$\begin{matrix}{{\Delta \; c} = {{I\frac{\phi \; {Mt}}{zFV}} = {j_{L}\frac{A\mspace{11mu} \phi \; {Mt}}{zFV}}}} & (2)\end{matrix}$

which shows the importance of maintaining a large cathode area, a highlimiting current density (j_(L)) and a high current efficiency.

The mass transport coefficient k_(m) is defined as:

$\begin{matrix}{k_{m} = {\frac{j_{L}}{zFc} = \frac{I_{L}}{AzFc}}} & (3)\end{matrix}$

where j_(L) is the limiting current density, I_(L) is limiting currentand c is the concentration of metal ions in the bulk solution. Combiningequations (1) and (3) gives an expression for the maximum rate of metalion removal:

$\begin{matrix}{\frac{w}{t} = {{ck}_{m}{AM}}} & (4)\end{matrix}$

which clearly indicates the importance of maintaining high currentefficiency, mass transport, cathode area and bulk concentrations ofmetal ions. Where it is particularly desirable to obtain thick andsmooth films of a metal from a dilute electrolyte, the inventorsconcluded from the observations above that mass transport at theelectrode interface is critical and should be increased as much aspossible. Among other possible mechanisms, mass transport can besubstantially increased by increasing turbulence, and/or providing ahigh flow velocity at the cathode.

EXAMPLE 1 Recovery of Lead from Contaminated Soil

The inventors tested a configuration in which lead-contaminated soil wasplaced in large bins and washed with an electrolyte containing EDTA toform the catholyte from which the lead was subsequently recovered in aelectrolytic cell. The treated electrolyte was then used to re-wash thelead-contaminated soil, thereby removing more lead from the contaminatedsoil. The process of soil-wash followed by recovery of lead in theelectrolytic cell was repeated as long as necessary to reduce theconcentration of lead in the soil to the desired value.

The electrolytic cell was designed as a classical tank electrolyzer witha pumped flow system as depicted in FIG. 1 to ensure generate high masstransfer conditions in the cell. Previous experiments indicated that arelatively low flow would have reduced the current at which one canplate smooth film deposits so that the lead could easily be harvestedfrom the cathodes.

The anode, here a lead-antimony alloy, was placed in a box which had twomembranes fitted as windows either side of the anode. The box was filledwith electrolyte, 5-10% sulfuric acid, which was pumped around the boxand back to an exterior anolyte tank so that the oxygen generated at theanode could escape to the atmosphere. Circulating anolyte provided somecooling effect so that continuous operation was possible.

The cathodes were placed exterior to the anode box opposite the membranewindows. The catholyte, the lead EDTA rich electrolyte from the soilleaching, was pumped from a holding tank into the outside box andallowed to overflow into a third box and back to a second holding tank.The catholyte was subject to several passes through the electrolyticcell until the lead concentration in the electrolyte reached a pointwhere it was denuded enough for the electrolyte to be successful as aleaching agent again. Note that the EDTA becomes a free acid or a mixedcalcium/sodium solution depending upon the pH of the reaction and theother cations present in the system.

Graph 2 below shows the concentration of lead in the electrolyte forseveral passes through the contaminated soil. Each curve in this graphrepresents a separate soil treatment. Each point in a selected curverepresents a separate pass through the electrolytic cell. Following thefirst treatment of the soil, the lead concentration in the EDTA solutionwas about 8000 ppm. This was reduced to about 5500 ppm after four passesthrough the electrolytic cell; the lead was recovered as foil plated onthe cathodes (FIG. 3). Following the second soil treatment, the leadconcentration in the EDTA increased to 13,000 ppm, which was furtherreduced to 8000 ppm after four passes through the cell, before againbeing used to treat the soil, thus demonstrating that the EDTA solutioncould be re-used. By the eighth soil treatment, the amount of lead inthe soil had been reduced to the point where the concentration of leadin the EDTA solution immediately following the soil treatment was about2500 ppm. This was reduced to about 1000 ppm after five passes throughthe electrolytic cell. As was the case with the higher leadconcentrations, the lead was recovered as foil plated on the cathodes.

Graph 3 below depicts the amount of lead plated on the cathodes andcumulative plating efficiency throughout the test. The faradaicefficiency of lead plating ranged from 70% at the higher leadconcentrations to as low as 20% at the lower concentrations, however, inall cases the lead plate was obtained as foil. The cumulative efficiencywas about 57% throughout much of the plating operation, decreasing toabout 42% at the end of operations due to the lower lead concentrationin the electrolyte.

FIG. 3 is a picture of a cathode of an electrolytic cell according tothe inventive subject matter showing a partially-scraped lead plate.Apart from the inclusion of a membrane, the exemplary cell wassubstantially configured as a tank electrolyzer with a forced flow overthe cathodes. Various modifications to the depicted configurationclearly indicated that a forced flow directed over the inside spacebetween the cathode inner face and the membrane was critical to leaddeposition onto the cathodes as a smooth film.

While not wishing to be bound by any particular theory or hypothesis,the inventors contemplate that maintaining a high flow between the gapwill ensure high mass transport conditions as the diffusion layer isdisturbed by the flowing electrolyte. Without such flow, theconcentration of the metal would decline at the surface as electrolysisdepleted the concentration of target metal, which would allow formationof hydrogen gas and current limiting conditions described above.

Such configurations become particularly critical as the target metalconcentration declines during the remediation process. Formation ofdendritic deposits that are generally difficult to remove from a cathodeand often threaten the membrane or cause other problems (e.g.short-circuiting). In contrast, the present cell configuration allowedthe inventors to deploy an inexpensive, self-contained, and portablesystem to contaminated sites. Exemplary cells were operated day andnight with minimum attention and formed smooth metal films that could beeasily removed as plated lead from the cathode. Moreover, the exemplarycells allowed lead deposition from much weaker solutions (with respectto lead concentration) that would be viable with common tankelectrolyzers. In still further advantageous aspects, contemplated cellswere also operated under current limiting conditions and above tofurther deplete the electrolyte of the metal. Consequently, it shouldalso be recognized that contemplated cells may be operated underconditions to produce metal deposits in a form other than a smooth film(e.g., in form of a powdery or granular deposit, or in form ofdendrites.

EXAMPLE 2 Reduction of Lead-EDTA & Copper-EDTA Complexes with ConcurrentOxidation of EDTA

A four-chamber electrolytic cell comprising two carbon felt electrodes,one used as anode the other as a cathode, was assembled. The carbon-feltelectrodes were fabricated by attaching a porous carbon felt onto atitanium mesh surface. A NAFION™ ion-exchange membrane was used toseparate the two halves of the cell. The cell was configured so that theelectrolyte was pumped from a reservoir into the chamber in front of theelectrode, (i.e. between the electrode and the membrane), flowed throughthe porous electrode, into the chamber behind the electrode, and thenreturned to the reservoir.

One kg of soil containing about 1600 mg/kg lead and other metals(primarily copper, zinc and iron) was stirred with 10 liters of a 0.1 MEDTA solution and mixed for 24 hours. The slurry was filtered toseparate the soil from the treatment liquor. The soil was washed withthree pore volumes of water and drained. Approximately three liters ofthe treatment liquor, now containing low concentrations of copper-EDTA,iron-EDTA, zinc-EDTA and lead-EDTA complexes was placed in a tank andfed to the cathode side of the electrolytic cell described above. Asecond part of the treatment liquid, also about three liters, was placedin a second tank and fed to the anode side of the cell.

Graph 3 below shows the concentrations of copper, lead, zinc and iron inthe EDTA solution as a function of treatment time in the cell. The cellwas operated at a current density of about 100 A/m². The copperconcentration was reduced from 260 mg/l to non-detect (less than 0.1mg/l) in less than an hour at better than 90% faradic efficiency. Leadwas plated once all of the copper has been plated; the leadconcentration was reduced from 190 mg/l to less than 0.7 mg/l in abouttwo hours, corresponding to about 20% faradic efficiency. Iron and zincdo not plate under the conditions of this experiment (the presence ofEDTA interferes with plating of iron; the presence of iron interfereswith plating of zinc).

Graph 4 below shows the decrease in free EDTA concentration (i.e. EDTAthat is not complexed with metals) with time of operation of the cell.Note that at the pH of this test, between 4 and 6, the prevalent form ofEDTA is the divalent H₂EDTA²⁻ anion. The concentration of EDTA wasmonitored titrimetrically, by measuring its ability to complex astandard solution of ZnSO₄. Consequently, the concentration of EDTAshown in the figure is actually the concentration of all species thatwill complex with zinc. It is likely that these include some of theinitial daughter ions, which is why the rate of loss of EDTA appears toincrease after five hours.

This example illustrates how the residual lead in the electrolyte afterthe operation of the main high flow cell, is removed to very low limitsas plated lead onto a very high surface area cathode. As this solutionis to be disposed of rather than recovered, it is essential that theelectrolysis removes lead completely at the minimum cost for theoperation. It is further important to destroy any remaining complexingagents to avoid solubilization of other toxic metals. As this process isof no economic advantage, efficiency and operating cost are the mainconsiderations provided the efficacy of the operation is notcompromised. The high surface area divided cell meets these criteria.

EXAMPLE 3 Stabilization of Lead in Treated Soil

A final requirement is to remove or immobilize any remaining lead in thesoil such that it will pass any leaching process after the remediationprocess is complete. The following example demonstrates this by the useof ferric chloride solution. This stabilizer was chosen because iron isbeneficial in soils and is benign. Therefore, a suitable method ofimmobilizing lead ions in soil previously treated with a complexingagent will comprise a step of admixing a ferric chloride containingsolution to the soil. Further, some iron is lost in the process andshould be replaced. Other washing agents have been used successfully,hypochlorite, lignin sulfate, calcium chloride, calcium sulfide etc. Theiron chloride example is given here to illustrate the method.

Soil containing about 1600 mg/kg lead and other metals (primarilycopper, zinc and iron) was stirred with a 0.1 M EDTA solution at 10%solids and mixed for 24 hours. The slurry was filtered to separated thesoil from the treatment liquid. The soil was washed with three porevolumes, PV, of water and allowed to drain. (Note: one PV ˜0.2 ml/gsoil). Following this step, the soil was treated with approximatelythree PV of 0.1 M ferric chloride solution. The slurry produced wasstirred on a magnetic stirrer for two hours, and then filtered torecover the soil. The soil was then washed with another three porevolumes of water, before finally being air dried until it reachedapproximately the same moisture content as the original soil.

At each stage of the process, a sample of the soil was set aside foranalysis to determine the effectiveness of the treatment. Analyticalmethods used were the Japanese test method for total lead (digestion ofthe soil in 1 M HCl for two hours, 33.3 ml solution/g soil, using 6 to 7g of soil) and a modified version of the Japanese Elution Test forleachable lead (agitation of 50 g soil with 500 ml of pH 6.0 HClsolution for six hours). The modification on the Japanese Elution Testprocedure was that the eluant solution was prepared by serial dilutionof a 10⁻³ mol dm⁻³ HCl solution (using 18.4 MΩ DI water), until HClconcentration was nominally 10⁻⁶ mol dm⁻³ rather than the proscribedeluant, which is DI water adjusted to a pH between 5.8 and 6.3 viaaddition of HCl. The variation was used because pH meters and teststrips do not provide accurate pH readings in high purity (lowconductivity) solutions.

Treatment of the soil in the manner described above reduced the totalamount of lead in the soil from about 1300 mg/kg initially to between100 and 140 mg/kg, a reduction of approximately 90%. This treatment metthe required standard of the Japanese total lead test, which is for thelead concentration to be less than 150 mg/kg.

Table 1 below shows the results of the elution test. Following theinitial treatment with EDTA and the first water wash, the amount ofleachable lead in the soil, primarily in the form of the solublelead-EDTA complex in the soil pore-water, was increased by an order ofmagnitude. The secondary treatment step, using FeCl₃, reduced the amountof leachable lead by approximately two orders of magnitude, and thesubsequent final wash with water reduced the amount of leachable leadfurther, to reach the required standard.

LEACHABLE SAMPLE DESCRIPTION LEAD, mg/L Initial Soil Sample 0.3 PrimaryTreatment (EDTA + water wash) 4.7 Secondary Treatment (0.1 M FeCl₃, nowater wash) 0.027 Secondary Treatment (0.1 M FeCl₃, + water wash) 0.009Standard to pass test 0.010

Thus, specific embodiments and applications of electrochemical soilremediation have been disclosed. It should be apparent, however, tothose skilled in the art that many more modifications besides thosealready described are possible without departing from the inventiveconcepts herein. The inventive subject matter, therefore, is not to berestricted except in the spirit of the appended claims. Moreover, ininterpreting both the specification and the claims, all terms should beinterpreted in the broadest possible manner consistent with the context.In particular, the terms “comprises” and “comprising” should beinterpreted as referring to elements, components, or steps in anon-exclusive manner, indicating that the referenced elements,components, or steps may be present, or utilized, or combined with otherelements, components, or steps that are not expressly referenced.

1-26. (canceled)
 27. A method of operating an electrolytic cell,comprising: positioning a separator between an anode surface of an anodeand a cathode surface of a cathode in an electrolyzer such that a flowpath for an electrolyte through a cathode compartment is formed;providing the electrolyte to the cathode compartment, wherein theelectrolyte comprises a metal that is optionally in complex with acomplexing agent; positioning the separator and the cathode surface suchthat the electrolyte can be pumped through the cathode compartment at apredetermined flow velocity that provides a Reynolds number of at least2000; and pumping the electrolyte through the cathode compartment at arate that is at least the predetermined velocity along the flow path,and while pumping the electrolyte at the rate, applying a potential tothe cathode in an amount effective to allow deposition of the metal ontothe cathode surface as a smooth film at non-current limiting conditions.28. The method of claim 27, wherein the metal is present at aconcentration of below 5000 ppm.
 29. The method of claim 27, wherein thepotential is selected such that current density in the flow path isproportional to a concentration of the metal concentration and theReynolds number.
 30. The method of claim 27, wherein the flow path is anupward flow path through the cathode compartment.
 31. The method ofclaim 27, wherein the flow path designed such that at least 80 vol % ofthe electrolyte in the cathode compartment pass between the cathodesurface.
 32. The method of claim 27, wherein the step of pumpingcomprises recirculating the electrolyte through the cathode compartment.33. The method of claim 27, wherein the electrolyzer comprises at leastone of a jet, a protrusion, and a funnel that is configured to induce orincrease turbulent flow of the electrolyte.
 34. The method of claim 27,further comprising a step of operating the electrolyzer under currentlimiting conditions.
 35. The method of claim 27, wherein the cathodecomprises a carbon felt electrode.
 36. The method of claim 35, whereinthe flow path is configured such that the electrolyte flows through thecarbon felt electrode.
 37. The method of claim 36, wherein the flow pathis configured such that the electrolyte first flows between the cathodesurface and the separator, then flows through the cathode, and thenleaves the cathode compartment.
 38. The method of claim 37, wherein theelectrolyte is recirculated to the cathode compartment.
 39. A method ofoperating an electrolytic cell comprising: positioning an anode and acathode in an electrolyzer, wherein the cathode is in electrical contactwith an electrolyte that includes a metal at a concentration of lessthan 5000 ppm, wherein the metal is optionally in complex with acomplexing agent; and pumping the electrolyte along a flow path betweenthe anode and the cathode at a flow velocity and cathode potential atwhich the metal is plated onto the cathode in form of a smooth filmunder non-current limiting conditions.
 40. The method of claim 39further comprising pumping the electrolyte at a second flow velocitythat is greater than the flow velocity, wherein the metal is plated ontothe cathode at the second flow velocity in a form other than the smoothfilm.
 41. The method of claim 40 wherein the form other than the smoothfilm is a powdery deposit or a dendritic form.
 42. The method of claim39 wherein the metal is present in the electrolyte at a concentration ofless than 500 ppm.
 43. The method of claim 39 wherein the electrolyte isrecirculated, and wherein the metal is selected from the groupconsisting of copper, lead, and zinc.
 44. The method of claim 39 whereinthe cathode comprises a carbon felt electrode.
 45. The method of claim44 the flow path is configured such that the electrolyte flows throughthe carbon felt electrode.
 46. The method of claim 45 wherein the flowpath is configured such that the electrolyte first flows between thecathode surface and the separator, then flows through the cathode, andthen leaves the cathode compartment.